A new constraint on the rise of oxygen

Oxygen is essential to life on Earth today, but that wasn’t always the case. When cyanobacteria first started churning out O₂ as a byproduct of photosynthesis, more than 2 billion years ago, it was actually poisonous to most other species alive at the time, and a mass extinction followed. The evolution of atmospheric oxygenation is thus critically entwined with the evolution of life.

The rise of O₂ to its present concentration was neither a single sudden jump nor a smooth and steady climb. Rather, the concentration grew in fits and starts, stalling or even declining for hundreds of millions of years in between. The exact trajectory is poorly understood, though, because geological records of the ancient atmosphere are rare.

Now Princeton University’s Clara Blättler and an international team of collaborators have opened a new window into an important period in atmospheric history. They analyzed a collection of mineral samples drilled from deep under the shore of Lake Onega (shown in the photo), 300 km northeast of Saint Petersburg, Russia. The minerals were deposited during a sustained period of seawater evaporation 2.0 billion years ago, so they constitute a record of the ocean’s composition at that time. Of particular interest to oxygenation history is the water’s concentration of sulfate ions. The SO₄²⁻ was produced when atmospheric O₂ reacted with iron pyrite, or FeS₂, and it’s a potent oxidizing agent in its own right.

The Onega samples are by far the oldest intact marine salts ever found. In most cases, water seeps into a salt deposit, gradually dissolves the minerals, and replaces them with others. When that happens, the shapes of the replacement crystals still contain some information about the original salts. But they provide only a weak constraint on the ancient seawater’s composition.

By the good fortune of some unknown geological mechanism, part of the Onega deposit was perfectly protected from water for its entire history. Blättler and colleagues could therefore study not only the salts’ chemical composition but also the constituent elements’ isotopic ratios, which contain information about the crystallization kinetics. From those measurements, the researchers concluded that the marine sulfate concentration was at least 10 millimoles per kilogram, or more than 30% of its present-day level. That’s a surprisingly high value for 2.0 billion years ago, just a few hundred million years after the first appreciable accumulation of O₂ in the atmosphere, and it suggests that Earth’s transition to an oxygenated environment may have been more abrupt than previously thought. (C. L. Blättler et al., Science, in press, doi:10.1126/science.aar2687 .)